1. Field of the Invention
This invention relates to a process and an apparatus for measurement of small amounts of diffusible hydrogen within steels or weld metals.
2. Description of Prior Art
A small amount of hydrogen exists in steels or weld metals, and it is classified as residual hydrogen or diffusible hydrogen depending upon its behavior. It is known that the solubility of hydrogen in steel near its melting point is approximately 25 ppm at 1800.degree. C. under P.sub.H.sbsb.2 =1 atm. If such hot steel is quenched, the solubility of hydrogen suddenly drops and hydrogen gradually moves out of the steel crystal lattice. However, as the temperature of the steel drops, the hydrogen effusion rate is reduced, and the rate is 10.sup.-4 14 10.sup.-6 cm.sup.2 /sec. at room temperature. If a portion of the diffusible hydrogen gathers at some part of the steel lattice, this may cause cold cracking. Now, cold cracking of a weld joint can be considered as follows. Hydrogen which has effused from a deposited metal during cooling before completion of solidification immediately after welding does not affect the occurrence of cracks, while hydrogen which effuses after solidification does adversely affect the occurrence of cracks. In order to determine whether there is a strong relationship between retained diffusible hydrogen in the weld metal after quenching and the occurrence of cracks, hydrogen content has been measured with respect to welding material of high hydrogen level, thus the above mentioned consideration is correct with inaccurate measurement of diffusible hydrogen. Based on these results, the total amount of diffusible hydrogen can be accurately measured by measuring a predetermined ratio of diffusible hydrogen using a constant time period from completion of welding to start of coolng and a constant cooling rate. Accordingly, the presently available standardized methods, the JIS method and the IIW method, strictly restrict cooling conditions.
The above noted JIS method is JIS Z 3113-1975, in which a sample piece which has been cooled under restricted conditions is immersed in glycerine and effused gas displaces glycerine in a burette then the burette; scale is read. While, in accordance with the IIW method, mercury is used as collector liquid instead of glycerine as in the JIS method. However these methods have the following problems, respectively.
(1) glycerine is apt to absorb hydrogen, so that hydrogen recovery is 50-75% compared wth that of the IIW method.
(2) glycerine is highly viscous and the effused gas rising velocity in glycerine is approximately 0.01% of that in mercury. So it is difficult to collect the hydrogen bubbles into one part of the burette.
(3) mercury is toxic and very dangerous to human health.
(4) mercury prevents gas clinging to the inner wall of the glass tube from ascending.
(5) collector liquids in both methods are apt to include gas which clings to the sample surface.
(6) In addition to H.sub.2 gas, small amounts of CH.sub.4 and CO gas are also generated from the sample piece. Furthermore, N.sub.2 gas and O.sub.2 gas which have been clinging to the sample surface are included as well. Thus, reading the burette scale only makes it possible to determine the total amount of gas generated, but an accurate measure of diffusible hydrogen cannot be obtained.
(7) In the case of welding using a low hydrogen welding rod which is presently being developed, less than 1 ml diffusible hydrogen is included in 100 g weld metal. None of the above noted methods can achieve accuracy in this case.
In view of the above mentioned detects, the vacuum extraction method (gas burette extraction method) in which no collector liquid is used has been developed. However, in vacuum extraction leakage sometimes occurs, and air mixed with diffusible hydrogen may also be measured. Moreover, measurement error may occur due to temperature change, and this method is not satisfactory, either. Still further, even in this method, mercury has to be used for moving gas, which is not satisfactory from a safety stand point.
Recently, a technique for measurement of hydrogen with a gas chromatograph has been proposed in "Welding Research International Vol. 6 No. 3 pp 1-10". In accordance with this technique, a sample piece cooled according to the IIW method is inserted into an enclosed collector cylinder, and a portion of the diffusible gas within the collector cylinder is taken out and introduced into a gas chromatograph, then analyzed by recorder, thus a very small amount of hydrogen is measured with relatively high accuracy. However, in this method, a predetermined portion of sample gas has to be taken out by keeping the temperature and pressure of sample gas constant, because the volume of the gas is easily affected by gas temperature and pressure; but this is technically very difficult. And if it could be done, the analysis accuracy is still not satisfactory. Further, the apparatus is complicated and special skill and various corrections depending upon measurement conditions will also be necessary. Thus this method is not practical.